Before running the Volume Optimization, make and test a backup. Please make sure your backup strategy does not require manually inspecting each of the files to be replaced by the backup should it be required. Run the Volume Structures test to check the disk directory, and run the Surface Scan to check for unremapped bad blocks. Pre-existing disk directory errors and unremapped bad blocks are the two major causes of failed optimization.

In some cases a Mac may go to sleep during Optimization.  This should not be the case because optimization should ensure that the computer is seen as active. The solution to that potential problem (which affects few users) is to use the Energy Saver pane of System Preferences (in the Hardware section) to make sure that the computer never goes to sleep, and then if you wish, change the settings to their original values once optimization is complete.

If the Volume Optimization crashes or must be forced to quit, the consequences depend on exactly what the optimizer was doing when it stopped. The consequences range from nothing, to loss of a file, a folder, or the entire volume (in the rare case in which a piece of the disk directory itself was being moved when the optimizer stopped). Use of a UPS device to ensure a steady supply of electricity is highly recommended.